Avoid compiling dev-dependencies with minimal versions

Works around an issue where no version of `glium` actually depends on
semantically valid crates with `minimal-versions` enabled. Tests and
examples are still compiled and/or ran on other configurations.

Closes: #113

.travis.yml

@@ -10,4 +10,4 @@ matrix:
 script:
   - cargo build -v $FLAGS
   - cargo doc -v $FLAGS
-  - cargo test -v $FLAGS
+  - if [ "$FLAGS" != "-Z minimal-versions" ]; then cargo test -v $FLAGS; fi